* https://en.wikipedia.org/wiki/Native_Americans_in_the_United...
I guess an American-British would be somebody who immigrated to GB from the US in that convention, but actually I’m not sure what convention they use over there.
Anyway in this case they aren’t talking about anybody who’s immigrated at all so it is pretty much unrelated.
People with multiple nationalities or who grew up in a different country than their nationality very often describe themselves in terms of a single nationality, often the place they spent most of their childhood. "I'm Argentinian" "Oh, was it difficult to get a visa to come here?" "Well, I have an Italian passport because of my mother."
Americans describing themselves as British-American or whatever seems strange to us, since we can recognize typical British people but aren't seeing any of it in the American.
Even nailing it down to XYZ country in Europe is a bit of a stretch, as the European genepool isn't the most diverse thing in the world.